This application is designed to address the hypothesis that elevations in the lesion equilibrium in mitochondrial DNA (mtDNA) following oxidative damage is an early key factor that sets in motion the cascade of events that ultimately leads to the phenotypes associated with aging and certain neurodegenerative diseases. Specifically, we will focus on factors that are involved in disruption of the normal lesion equilibrium in mtDNA which exists between the formation of oxidative damage and its subsequent removal by repair processes. Additionally, we will explore some of the consequences resulting from changes in the lesion equilibrium. This will be accomplished through the pursuit of four specific aims. The first and second aims are designed to determine how repair mechanisms in neurons affect the re-establishment of the normal lesion equilibrium in mtDNA. These studies will employ cultures of different types of neurons to answer questions relating to the hypothesis that the ability of neurons to repair lesions in their mtDNA is a key component in the maintenance of a normal lesion equilibrium. Defects in this repair allow the lesion equilibrium to rise out of control and initiate some of the phenotypes associated with aging and neurodegenerative diseases. The third aim will evaluate the effects of recombinant DNA repair proteins targeted to the mitochondria. These studies will use primary cultures of neurons that are transfected with genes that encode mtDNA repair enzymes. These proteins will modulate mtDNA repair capacity and directly alter the lesion equilibrium in mtDNA act as an early key factor that ultimately leads to cellular dysfunction and death. The final aim is structured to determine regional differences in levels of oxidative damage in mtDNA in transgenic animal model of chronic oxidative stress. These studies will use these mice to explore the hypothesis that specific aims in the brain are more vulnerable to oxidative damage in mtDNA. This increased damage is predominantly in certain nucleotide "hot spots" which ultimately become mutated and contribute to phenotypes associated with aging and the pathogenesis of neurodegenerative disorders. When successfully completed, these studies will provide a more thorough understanding of the role that mtDNA damage and its subsequent repair can play in aging and the pathogenesis of some neurodegenerative diseases.